Here’s the practical breakdown.

1) Block storage

What it is: Raw storage volumes presented like a disk to a server/VM. You format it with a filesystem and the OS manages it.

Best for:

• Databases
• VM boot disks
• High-performance transactional workloads
• Apps that need low latency and fine-grained read/write control

Pros:

• Lowest latency and best IOPS of the three
• Full filesystem control
• Works well with traditional apps that expect a local disk
• Easy to tune for performance

Cons:

• Usually attached to one machine at a time
• Sharing between multiple servers is harder or not supported
• You manage filesystem, mounting, expansion, snapshots, etc.
• Less convenient for scale-out shared access

Cloud examples: EBS (AWS), Persistent Disk (GCP), Managed Disks (Azure)

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2) File storage

What it is: Shared network file system exposed via protocols like NFS or SMB. Looks like a mounted directory tree.

Best for:

• Legacy apps expecting shared folders
• Home directories
• Content repositories
• Web app uploads/assets
• Multiple servers needing concurrent access to the same files

Pros:

• Easiest replacement for local disk in many legacy apps
• Shared access from multiple instances
• Familiar POSIX-like directory/file semantics
• Good fit when the app expects paths, files, and directories

Cons:

• Generally slower than block storage
• More network latency
• Performance may be limited by throughput and IOPS tiers
• Can become a bottleneck for highly transactional workloads
• Metadata-heavy operations can be slower

Cloud examples: EFS (AWS), Filestore (GCP), Azure Files

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3) Object storage

What it is: Storage for objects accessed via API over HTTP, not mounted as a filesystem by default.

Best for:

• Backups
• Media
• Logs
• Static assets
• Data lakes
• Archival and long-term storage
• Large-scale unstructured data

Pros:

• Very durable and highly scalable
• Usually cheapest per GB
• Virtually unlimited capacity
• Great for sharing data across apps and regions
• Lifecycle policies, versioning, replication, and archival tiers are common

Cons:

• Not a real filesystem in the traditional sense
• Higher latency and different access model
• No real file locking or rename semantics like POSIX
• Not suitable for apps that require random updates to files in place
• Often requires app changes to use API calls instead of filesystem operations

Cloud examples: S3 (AWS), Cloud Storage (GCP), Azure Blob Storage

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Main tradeoff dimensions

Performance

• Block: best
• File: middle
• Object: lowest for traditional file-like access

Sharing / concurrency

• Object: best at scale for many clients
• File: good for shared directory access
• Block: usually single-attached, not shared

Compatibility with legacy apps

• File: best for apps expecting mounted directories
• Block: best for apps expecting a local disk and single-host filesystem
• Object: worst unless the app is rewritten

Cost

• Object: usually cheapest
• File: typically more expensive than object
• Block: often more expensive per GB than object, but depends on performance tier

Durability / scale

• Object: best
• File: good, but usually more operationally constrained
• Block: durable in cloud, but not designed as shared durable storage for many consumers

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For a legacy app migrating off local disks

A rough rule of thumb:

• If the app expects C:\path or /var/lib/app and needs a mounted disk: start with block storage
• If multiple app servers need to access the same files: use file storage
• If the data is mostly blobs, documents, media, backups, or logs: use object storage
• If the app does lots of small random writes and needs low latency: block is usually safest

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Common migration pattern

Many legacy apps end up with a hybrid setup:

• Block storage for app binaries, databases, and writable local state
• File storage for shared uploads or working directories
• Object storage for backups, archives, logs, and static content

If you want, I can also give you:

1. a decision tree for choosing between them, or
2. a migration checklist for a legacy app moving from local disk to cloud storage.